2013 Environmental Monitoring Program Lakes Anne, Thoreau, Audubon and Newport Bright Pond and Butler Pond

Transcription

1 2013 Environmental Monitoring Program Lakes Anne, Thoreau, Audubon and Newport Bright Pond and Butler Pond Prepared for: The Reston Association Sunset Hills Road Reston, VA Prepared by: Kevin Laite Aquatic Environment Consultants, Inc. P.O. Box 307 Scotland, PA January 2014

2 Table of Contents Section Page Synopsis 3 Sampling Procedures and Analytical Methodology 8 Parameters Measured During the 2013 Monitoring Program Dissolved Oxygen (DO) 9 Dissolved Oxygen Saturation 10 Temperature 10 ph 10 Conductivity 11 Total Phosphorus 11 Secchi Disk Transparency 11 Chlorophyll a 12 Phytoplankton 12 Zooplankton Climatological Conditions 16 Lake Anne Chemical and Physical Parameters 20 Biological Monitoring Results 21 Lake Thoreau Chemical and Physical Parameters 43 Biological Monitoring Results 44 Lake Audubon Chemical and Physical Parameters 66 Biological Monitoring Results 67 Lake Newport Chemical and Physical Parameters 89 Biological Monitoring Results 90 Bright Pond Chemical and Physical Parameters 112 Biological Monitoring Results 112 Butler Pond Chemical and Physical Parameters 117 Biological Monitoring Results 117 Trophic State Indices 122 Recommendations 131 Literature Cited 134 1

3 Appendix A: Lake Anne Monitoring Data Appendix B: Lake Thoreau Monitoring Data Appendix C: Lake Audubon Monitoring Data Appendix D: Lake Newport Monitoring Data Appendix E: Bright Pond Monitoring Data Appendix F: Butler Pond Monitoring Data Appendix G: Monthly Climatological Data Summaries 2

4 Synopsis This report discusses the results of the environmental monitoring program conducted from April through September of 2013 for lakes Anne, Thoreau, Audubon and Newport. The Reston Association has been monitoring water quality in the lakes since 1981; Lake Newport was added to the monitoring program in Two additional water bodies were added to the monitoring program in 2003 Bright and Butler Ponds. Water quality parameters were evaluated during the July visit for Bright Pond and Butler Pond. Only one visit was made to these ponds this sampling season. The 2013 monitoring was conducted by Aquatic Environment Consultants, Inc., following procedures established in previous years to ensure the data collection was compatible with previous data. By using similar methods and comparing data, long-term trends can be studied in the water quality of the lakes. The precipitation this season was below average for four out of the six months during sampling. July received the greatest precipitation at 7.3 inches (3.6 inches above normal), while September was the driest at 1.6 inches (2.3 inches below normal). Temperatures were slightly warmer than normal from April through July, but August and September were just the opposite. April was surprisingly the warmest at 1.7 F above average. The second week of April saw highs of almost ninety degrees Fahrenheit. The rainfall preceding a sampling event will often affect the total phosphorus values. The phosphorus values are often elevated due to the increased runoff. This was not the case this year as July received the greatest precipitation but total phosphorus was highest overall in June when much less rain fell. In general, some water bodies seem to maintain better water quality in years that are wet while others are just the opposite. The weather has definite impacts on water quality, but it seems that one year it can be positive and for some reason the next year it is negative. Overall, ambient temperatures during sampling this year were above normal and kept epilimnetic temperatures above average on all the lakes. Conductivity levels were all up from last season. All lakes were above corresponding long-term averages for the season with regard to conductivity. Phytoplankton densities were varied throughout the season and unfortunately Lake Thoreau had the second highest seasonal average again. Algal populations remained relatively low on all the lakes through the early season. By June, Audubon and Thoreau experienced higher algal densities. Overall, Lakes Audubon and Thoreau had the greatest phytoplankton concentrations this year. Lake Audubon again had the highest average for the season with 59,062 cells/ml. The highest monthly algal density belonged to Lake Audubon as well 105,236 cells/ml during August. The aeration system on Lake Anne remains off and six monthly algaecide applications were made to control the phytoplankton populations. This was the ninth year since aeration was discontinued and monthly algae treatments began. The result continues to be reduced algal density and biomass. The algaecide treatments continue to have a profound effect on the algal populations. The 3

5 seasonal density of phytoplankton for the last nine years is three times less than it was for the previous nine without treatments. Treatments are timed so that they do not immediately precede any sampling date. Figure 2 shows the dissolved oxygen and temperature profile for Lake Anne. When the aeration system was running, the profile was dissimilar to those of the other lakes. The thermocline occurred much deeper in the water column or was absent. With lack of aeration, the thermocline typically occurs around two to three meters (similar to the other lakes). Epilimnetic temperatures for Anne this year were warmer than most years and more than one degree Celsius above the historic average. The hypolimnetic temperatures were just the opposite. The hypolimnetic average was 12.9 C. This was one of the cooler years even compared to the years without aeration when the temperatures were lower. The average whole lake temperature was 17.2 C, similar to what it has been since the aeration was shut down and before aeration started. Epilimnetic percent saturation values continue to be higher than the long term average. The eight years with aeration brought the historic epilimnetic average down because the hypolimnion and epilimnion were mixed. Conversely, during those 8 years the whole lake and hypolimnetic averages were higher. Hypolimnetic saturation was similar to that observed during the recent past. Percent saturation of the entire lake was slightly lower compared to years pre and post aeration and still remains below the long-term average. The ph levels for 2013 in Lake Anne were above average in the epilimnion which also brought the whole lake average up. The hypolimnetic seasonal average was equal to the historic average of 6.6 and the whole-lake and epilimnetic ph levels were 7.0 and 7.5 respectively and corresponding historic averages are 6.8 and 7.0. Overall, conductivity levels were slightly elevated this year. The whole-lake average was 283µmhos/cm and the epilimnetic average was 247µmhos/cm. As always, the hypolimnetic conductivity was the highest. The seasonal average was 311µmhos/cm. The seasonal averages were all within 70µmhos/cm of their corresponding historic average. The seasonal average of 1.2m for Secchi depth was down half a meter from last year and 0.4m below the historic average. Water clarity was not affected by heavy blooms, yet stayed between one and two meters all season. As mentioned, the algal blooms that use to occur throughout the season and affect the chlorophyll, Secchi depth and possibly phosphorus values can be kept under control when treatments are performed. The season average of 10.4µg/L for chlorophyll was one of the higher values since treatments began though. The highest reading for chlorophyll occurred in June and was 22.1µg/L. Strangely, the value did not correspond to the highest algal density or biomass. Phosphorus levels were relatively low throughout the season ranging from a high of 0.037mg/L in June to a low of 0.005mg/L in July. June was the highest month in regards to phosphorus on all of the other lakes as well. Over three inches of rain fell in the two weeks prior to sampling in June, but over five inches fell in the two weeks prior to sampling in July when the phosphorus values were considerably lower. The 4

6 phosphorous levels were above the benchmark of mg/l that is associated with nuisance algal blooms during four of the six months this season, but algal density was only elevated during two months. The cyanophyta or blue-green algae were historically the dominant group of algae during the season. When treatments began, a key shift in that trend took place. Green algae have dominated on more occasions than not. This season followed that trend and the overall density was similar to years with treatments. The average density during the years with treatments is 8,707 cells/ml and this year was 9,422 cells/ml. The historic average is 17,625 cells/ml. The algal biomass was proportionately lower though. Lower biomass does not always coincide with lower density, because one species may be more numerous but physically smaller. The biomass average for the season was 1,752 µg/l compared to the average of 2,714 µg/l during the treatment years and long term average of 5,826 µg/l. Zooplankton density and biomass remained relatively low. The seasonal density was 29.6 organisms/l. Copepods and rotifers were the most numerous during most of the season and copepods were the greatest contributors to biomass. The zooplankton biomass average for the season was 41.2µg/L. A lack of Chaoborus always keeps the biomass lower than years when they are present. High numbers in past years were often the result of spring blooms of rotifers and/or protozoans. Pleasantly, rotifers were present this spring, just not as numerous as past years and protozoans were not as numerous as well. The temperature/dissolved oxygen profile for Lake Audubon began to show fairly distinct variation by June. Seasonal water temperatures for the epilimnion were above long-term averages. The whole-lake average for the year was 18 C which was equivalent to the historic average. Dissolved oxygen levels in the epilimnion and hypolimnion were both above average for the season. Hypolimnetic levels were not high enough however to raise the whole-lake above average for the season. The ph levels were all close to long-term averages. The conductivities were up this year for Audubon. The epilimnetic values for the year were approximately 40µmhos/cm above average and the hypolimnetic and whole lake values were about 20µmhos/cm above average. Secchi depth in the lake fell from 1.8 meters last year to 1.4 meters this year. The clarity fell below one meter in September and was as high as 2.5 meters in April. The seasonal mean chlorophyll a concentration in Lake Audubon was 15.1µg/L which was less than 2µg/L above the historic average. August and September were the months with the greatest phytoplankton populations. The algal density in August was 95,688 cells/ml and September had a density of 105,236 cells/ml. May and September brought the overall phytoplankton density up to 59,062 cells/ml which was about 40,000 cells/ml above the historic average. Algal biomass was also above average, but not to the same degree due to the smaller size of the blue-green algae present throughout the season. 5

7 While the algal populations were multiplying, the zooplankton were not as prolific. During the August and September blue-green blooms, there were only 7.7 and 9.4 zooplankton per liter present. A modest population of protozoans gave June the highest density of 68.5 zooplankton per liter. The zooplankton biomass was also highest in June, but due to the cladocerans present. The rotifers and copepods were the largest contributors to density for the season. Copepods and cladocerans accounted for most of the biomass. The seasonal zooplankton density as well as biomass was below normal. The seasonal epilimnetic temperature for Lake Thoreau this season was 1.6 C above average. Whole lake temperatures were 0.2 C below though. Oxygen levels in the hypolimnion remained relatively high until July. Lake Thoreau typically has the highest percent saturation in the hypolimnion for all the lakes (Lake Anne use to be the highest due to aeration). The hypolimnetic average for Thoreau was 25.5% and the next highest hypolimnetic percent saturation was 15.7% on Lake Audubon. The seasonal mean ph levels for the epilimnion were up slightly from last year and the hypolimnetic values were as well. As with Anne and Audubon, conductivities were all up this year. The seasonal mean conductivity levels in the epilimnion were 84µmhos/cm above average and the hypolimnetic values were 80µmhos/cm above average. The mean seasonal Secchi depth of 2.1 meters was the best of all the lakes, but lower than many observed in recent years. The Secchi depth was as high as 3.5 meters in April and May, but dropped to 0.8 meters in June. Similar to Secchi, Thoreau typically has had a fairly good average for chlorophyll a. The 2013 seasonal mean chlorophyll a concentration was 12.6µg/L which was not the lowest of the lakes. The chlorophyll has slowly crept up over the last three years. The main factor that generates low chlorophyll and enhanced clarity is low phytoplankton density. This year again though, Lake Thoreau had some of the higher phytoplankton densities. June had an algal density of 100,148 cells/ml (the monthly average to date is only 9,237 cells/ml for the month of June). The seasonal average this year was the highest to date at 45,424 cells/ml. For the last five years, the seasonal density has been higher than normal. This year, seasonal density was more than three times the historic average and the biomass was almost twice the average to date. Zooplankton density this season was similar to recent years, but still below average. Rotifers were present in moderation throughout the season. There was no spring bloom of protozoans to augment the zooplankton density early and no other group really thrived during the rest of the season to increase zooplankton density or biomass. As was the case in the other lakes, the copepods and cladocerans accounted for nearly all of the zooplankton biomass. With lower overall density and few Chaoborus this season, the biomass was 22.6µg/L. The Chaoborus have helped to maintain the long-term average over the years, but this season s average was well below the historic 212.4µg/L average. 6

8 Being the shallowest lake, Lake Newport maintains the warmest water temperatures overall. The 2013 whole lake average was 19.2, compared to Thoreau (the deepest lake) which was 15.5 C this season. Thermal stratification was present throughout most of the season, but was most distinct in July with 12.1 C difference between strata. During July, the epilimnetic average topped out at 29.2 C (85 F). The oxygen profile for Lake Newport was average this year. The hypolimnetic saturation was 12.9% which was just under the long term average of 13.6%. Due to proportionately lower epilimnetic oxygen levels, the whole lake saturation of 41.3% showed greater deviation from the historic average of 48.7%. Newport will tend to stratify at a slightly shallower depth and in general the hypolimnion is more void of oxygen than the epilimnion. Average ph values this season were all 0.3 to 0.4 units below historic values. The conductivity levels remain the lowest of all the lakes this season. The epilimnetic average was only 123µmhos/cm and the hypolimnetic average was 134µmhos/cm. By comparison, Lake Anne hypolimnetic average was 311µmhos/cm. The average clarity was down from last year, but better than many years past. The seasonal average was 1.7 meters this year compared to the historic average of 1.6 meters. The Secchi depth began at 2.5 meters in April and remained above 1 meter all season. Newport again experienced lower algal density this year which led to lower chlorophyll values as well. The season s greatest density was 9,696 cells/ml in July which corresponded to an algal biomass of 4,084µg/L and chlorophyll of only 6.9µg/L. Conversely, September had a density of only 1,738 cells/ml; a biomass of 3,200µg/L and this corresponded to a chlorophyll value of 26.1µg/L. So with lower density and lower biomass, the chlorophyll value was quite different in July. This season s algal density average of 3,758 cells/ml (second lowest to date) was well below the historic average of 29,696 cells/ml and the biomass was below average as well. The chlorophyll average of 12.6µg/L was below the current average of 16.6µg/L. Zooplankton density peaked early in the season during May for Lake Newport. Rotifers had the largest populations throughout the season. Chaoborus were present late season, but copepods and cladocerans consistently contributed the majority of the biomass throughout the season. The seasonal density of 41.7 zooplankters/l was below the historic average of zooplankters/l, but was similar to seasons over the past few years. The seasonal biomass was comparative to density with regards to historic averages. The trophic state indices for the lakes in 2013 showed nothing out of the ordinary. The chlorophyll TSI s for three of the lakes was up this year. Lake Newport had the worst chlorophyll TSI from last year. The TSI went from 49.4 to 55.4 this year. Chlorophyll values for Thoreau and Audubon resulted in TSI values of 55.4 and 57.2 respectively. Anne had the lowest chlorophyll TSI at As usual, better clarity made the corresponding Secchi TSI value for Thoreau the lowest of all the lakes. The Secchi TSI for Thoreau was The clarity of Audubon and Newport resulted 7

9 in TSI values of 55.1 and 52.3, while Anne had the worst clarity resulting in a TSI of Total phosphorus TSI values were actually the lowest overall this season. The phosphorus TSI for Lake Thoreau was lowest and Newport was the highest. Anne, Thoreau, Audubon and Newport had corresponding phosphorus TSI values of 48.1, 40.0, 47.4 and Lake Thoreau is typically the only lake with TSI values below 50. This year, Anne and Audubon had a TSI value below 50 as well. Using Carlson s index (Carlson, 1977) for TSI, a mesotrophic lake would have a TSI of Based on the values obtained in 2013, all lakes except Thoreau are out of that range for most values, but still within reasonable limits. The other lakes would fall into the category of eutrophic. This term comes from the Greek for well-nourished and describes lakes that have high photosynthetic activity and low clarity. 8

10 Sampling Procedures and Analytical Methodology The months of April through September were chosen as the time period for water sampling to cover the period of maximum biological activity in the lakes. Water samples and water quality data were collected once each month at approximately five-week intervals. The parameters that were used for the monitoring program are listed in Table 1 below. The sampling location in each of the lakes was what is believed to be the deepest point of each lake. These sampling sites were located in the center channel near the principal spillway of the lakes. Only one sampling site for each lake was selected. Due to the size of the lakes, one site on each lake was sufficient to obtain a representative sample of the water quality for the lake. Table 1 Water Quality Parameters Measured for 2013 Dissolved Oxygen (DO) Dissolved Oxygen Saturation (%) Temperature ph Conductivity Total Phosphorus Secchi Disk Transparency Chlorophyll a Phytoplankton Zooplankton The parameters in Table 1 are indicators of the health of a water body and the ability to support aquatic life. These parameters also help to determine a lake s trophic state and relate interactions between the chemical and biological components of a lake and the ecosystem. The analyses performed during the monitoring process were conducted in accordance with Standard Methods, Dissolved oxygen and temperature were measured using a YSI Model 57 meter at half meter depths to the deepest point on the lake. Conductivity was measured using an ExStik EC500 meter at one meter depths. An Oakton BNC ph meter equipped with a silver chloride electrode was used to measure ph at one meter depths. Water samples taken from incremental depths were collected with a Wildco beta plus horizontal water sampler equipped with a stainless steel messenger. In addition to the parameters listed above, the weather conditions were monitored on each sampling date as well as compiled weather data for the six-month monitoring period. Total phosphorous concentrations were measured from composite samples taken in the epilimnion of each lake at depths of 0, 1, 2 and 3 meters unless stratification occurred at a shallower depth. A duplicate composite sample was prepared each time. On some sampling days, composites were not pulled from all 4 depths, at which time the depth of the mixed layer was less than 3 meters. The use of duplicate composite samples should give a more accurate indication of phosphorous levels over the monitoring period. Water samples for total phosphorous were placed in sample bottles containing preservative and then stored on ice while in the field. The samples were delivered to the 9

11 lab for analysis the day of sampling. The samples were analyzed using the colorimetric ascorbic acid method (Standard Methods, 20 th Ed., Method 4500-P B.5 E). Biological characteristics of the lakes were ascertained through the analysis of chlorophyll a and the identification of major genera of phytoplankton and zooplankton. The composite water taken for the total phosphorous samples was also used to take samples for the chlorophyll a and phytoplankton analyses. A determined volume of water was filtered in the field for chlorophyll a analysis. The filter papers were then placed in glass vials and stored on ice while in the field until they could be frozen. Once frozen, the samples were shipped to Dr. Gregory Boyer of the Biochemistry Department, SUNY-ESF for analysis using the Welschmeyer fluorometric method (Welschmeyer, N.A. 1994). Samples for the identification of phytoplankton and zooplankton populations were collected following Standard Methods (Method B). The composite phytoplankton samples were preserved with two percent by volume M3 and stored in a cool, dark location until they were sent to Dr. Ken Wagner at Water Resource Services, Inc. for identification and enumeration. Phytoplankton were identified to the lowest practical taxon and counted using a Sedgewick-Rafter chamber and a microscope equipped with a Whipple grid. Zooplankton samples were collected over a 30 meter net tow from the entire water column using a 60 µm mesh plankton net. Zooplankton samples were preserved with fifteen percent by volume buffered formalin solution (Lind, 1979). The zooplankton samples were also identified and enumerated by Dr. Ken Wagner. Parameters Measured During the 2013 Monitoring Program Dissolved Oxygen (DO) The amount of oxygen present in the water and the profile of this oxygen throughout the water column are important indicators as to the health of a lake. By studying this one parameter, a large amount of information can be determined. The DO content of water results from photosynthesis, diffusion at the air-water interface and distribution by wind-driven mixing. The amount of oxygen produced through photosynthesis is related to the amount of plant and algal life and thus the productivity of the lake. The profile of the DO in the water column can give insight into the mixing patterns and effectiveness of mixing processes in a lake. The DO will fluctuate with changes in temperature and changes in photosynthetic activity and diffusion. Surface waters are often supersaturated with DO during daylight hours. Oxygen is used continuously by the pond biota in respiration, but during the day photosynthesis normally produces oxygen faster than it is used in respiration so that DO concentrations remain high. Phytoplankton die-off and sudden destratification of the water body can cause rapid oxygen depletion. If the DO falls below 4.0 mg/l most desirable aquatic organisms will be stressed and may even die. 10

12 Dissolved Oxygen Saturation Water containing the amount of DO which it should theoretically hold at a given temperature, pressure, and salinity is said to be saturated with oxygen. Likewise, waters containing less than or more than the theoretical concentration are said to be under saturated or supersaturated with oxygen, respectively. The degree of oxygen saturation of water is expressed as percent saturation and water saturated with oxygen is at 100 percent saturation. The amount of oxygen that can dissolve in water decreases with increasing temperature and salinity and with increased dissolved solids, therefore, dissolved oxygen saturation provides a better means of comparing oxygen concentrations from different sampling dates and depths in the water column. Temperature Sufficient and accurate temperature data are important. Temperature directly and indirectly exerts many fundamental effects on limnological phenomena such as lake stability, gas solubility and biotic metabolism. One of the most important relations of the temperature to water is the decrease in the solubility of oxygen in water as the temperature increases. Temperatures in a lake are a function of ambient air temperatures and the physical characteristics of the water itself. The turbidity of a water body can inhibit light from passing through the water column and warming the water. Light energy or the heat generated from the light is absorbed exponentially with depth, so most heat is absorbed within the upper layer of water. Since heat is absorbed more rapidly near the surface of a water body and the warm upper waters are less dense than cool lower water, bodies of water may stratify thermally. This occurs when differences in density of upper and lower strata become so great that the two cannot be mixed by wind action. ph The ph of a solution is a measure of its hydrogen ion activity and is expressed as the logarithm of the reciprocal of the hydrogen ion concentration. It is important to remember that a change of one ph unit represents a tenfold change in hydrogen ion concentration. The ph scale ranges from 1.0 to 14.0 standard units. A ph of 7.0 indicates neutral conditions, while waters with a ph less than 7.0 are said to be acidic and those with a ph greater than 7.0 are said to be basic. The ph of most natural waters falls in the range of 4.0 to 9.0, and much more often in the range of 6.0 to 8.0. The desirable range for fish production is 6.5 to 9.0. The acid death point for fish is around 4.0 or less. In water bodies, deviation from the neutral ph 7.0 is primarily due to the hydrolysis of salts of acids and bases. Dissolved gases such as CO 2, H 2 S, and NH 3 also have a significant effect on ph values. The majority of natural water bodies have a somewhat alkaline or basic ph due to the presence of carbonates. Values for ph and 11

13 the changes in these values are important, since they may reflect biological activity and changes in natural chemistry of waters, as well as pollution. Conductivity Conductivity or specific conductance is a measure of water s capacity to conduct an electric current. Conductivity is the reciprocal of resistance for which the standard unit is an ohm. Since conductivity is the inverse of resistance, the standard unit for conductivity is the mho. In low-conductivity natural waters, the standard unit is the micromho. Because the measurement is made using two electrodes that are one centimeter apart, conductivity is generally reported as micromhos per centimeter (µmhos/cm). Different ions vary in their ability to conduct electricity, but, in general, the greater the concentration of ions in natural water, the higher the conductivity. Temperature also affects conductivity. Conductivity will generally increase two to three percent per degree Celsius. For comparison of values, conductivity is usually corrected to one standard temperature which is most often 25ºC. The most useful information that can be gathered from conductivity readings is the estimation of the total concentration of dissolved ionic matter in the water, which in turn relates to water fertility. Total Phosphorous Phosphorous is a key metabolic nutrient and the supply of this element often regulates the productivity of natural waters. Total phosphorous is the sum of all forms of phosphorous present. Phosphorous is present in water in several soluble and particulate forms, including organically bound phosphorous, inorganic polyphosphates and inorganic orthophosphates. Orthophosphates, which are ionized forms of orthophosphoric acid (H 3 PO 4 ), are the simplest forms of phosphorous present. The ph of the water will affect the degree of ionization and thus the amount of orthophosphates present. The natural source of phosphorous to waters is from leaching of phosphate containing rocks and from organic matter decomposition. Additional sources are found in manmade fertilizers, domestic sewage and detergents. Inorganic and organic phosphates may reach waters through effluent and runoff. Phosphorous is lost from the water by chemical precipitation to sediment and by adsorption on clays or sediment with high ph and carbonate levels. Phosphorous is usually found in low concentration in natural waters, but is used readily by plants for growth. The element present in the lowest concentration relative to demand is the element limiting the process at a given time. This is why phosphorous is usually said to be the limiting factor of plant and algal growth and if found in excess is most likely to cause excessive plant or algal blooms. Secchi Disk Transparency Visibility is a measure of the depth to which one can see into the water. The Secchi disk is a simple device used to estimate this depth. The disk is a weighted circular plate, 20 cm in diameter, with a painted surface consisting of alternate opposing black and white 12

14 quarters. The disk is attached to a depth-calibrated chord attached to a ring in the center of the disk, so the disk is horizontal when lowered into the water. To determine the Secchi disk visibility, the disk is lowered into the water until the disk disappears and the depth is noted. The disk is lowered further then slowly raised until it is visible again and this depth is noted. The final Secchi depth is the average of these two readings. Secchi depth corresponds to the depth where light penetration is ten percent or less and approximates the lower level of photosynthetic activity. The transparency is based on the transmission of light through the water and is related to the amount of natural light, amount of inorganic suspended solids and the amount of organic suspended solids. The Secchi disk measures the turbidity of water. Plankton is usually the major source of turbidity, so Secchi depth can give an estimate of plankton density. When compared with data on chlorophyll a, particulate organic matter and phytoplankton counts, Secchi depth correlates most with particulate organic matter. Particulate organic matter is a measurement that includes living zooplankton and phytoplankton as well as dead organic particles. For northern lakes, a Secchi depth of greater than 30 feet is considered oligotrophic while the eutrophic lakes may have a reading of 3 to 4 feet or less during summer algal blooms (Moore, 1988). Secchi depths of less than two meters are usually considered undesirable for recreational lake uses and even lower values may indicate the onset of an algal bloom. Chlorophyll a Chlorophyll is a green pigment in algae and other green plants that is essential for the conversion of sunlight, carbon dioxide and water to sugar that may then be used as food. Chlorophyll a is a type of chlorophyll present in all types of algae, sometimes in direct proportion to the biomass of the algae. The values may also be used to characterize the age, structure, quantification of the phytoplankton and photosynthetic rates. Phytoplankton Phytoplankton is microscopic algae and microbes that float freely in open water. Phytoplankton occurring in lakes includes members belonging to one of the following taxonomic divisions: green algae (Chlrorophyta), blue-green algae (Cyanophyta), diatoms (Bacillariophyta), yellow-green and golden-brown algae (Chrysophyta) and dinoflagellates (Pyrrhophyta). The chlorophyta or green algae do not compose, for the most part, a significant portion of food chains nor do they affect other organisms positively or negatively. These organisms have not been found to kill fish or edible invertebrates and therefore do not receive much public notice. They are primarily fed upon by rotifers and protozoa, but usually environmental constraints can keep their populations in check. Nitella, Chara and Spirogyra are some genera from this division commonly found in North American lakes. A bloom of a species from this division is not as common as other taxonomic 13

15 groups such as the blue-green algae. There are however some genera that may cause a green bloom such as Hydrodictyon, Volvox, Pandorina and Volvulina. Cyanophyta or cyanobacteria are commonly referred to as the blue-green algae because they are a group of bacteria that can photosynthesize and thus produce their own food. This division also has an advantage over other groups because the blue-greens have the ability to fix nitrogen, meaning they can utilize atmospheric nitrogen and store it and therefore exist even if levels of nitrogen in the water are low. The blue-green algae are probably the most commonly known group of phytoplankton known by the public due to the problems they may cause. Blue-green blooms have been associated with offflavor in fish, toxic substances, shallow chemical and thermal stratification, taste and odor in drinking water, phytoplankton die-offs and unsightly appearance on the water surface. The two most serious problems associated with excessive blue-green blooms are foul odor or taste of the fish or water and sudden, massive phytoplankton die-off. Buoyant blue-green algae often accumulate at the surface of a water body and form a green scum. If the scum is heavy or thick and dies of suddenly, fish kills may result from the depletion of oxygen following decay of the dead algae. Blue-green blooms are associated with high concentrations of nitrogen and phosphorous but not all water bodies with these characteristics may have a bloom. Some other associated reasons may be: 1. High concentrations of organic matter, 2. High concentrations of nitrogen and phosphorous at low CO2 levels and high ph and 3. Excretion of antibiotics by bluegreen algae which inhibit other algae and favor blue-greens (Boyd, 1981). Some bluegreen algae associated with taste and odor changes in water are Anabaena, Microcystis, and Aphanizomenon. One other genus of blue-green known as Lyngbya is associated with the skin irritation commonly known as swimmer s itch. The Bacillariophyta are the diatoms. This group looks like glass structures when viewed under a microscope due to their silica shells. Diatoms are among some of the most important aquatic microorganisms today. They are abundant in both the plankton and sediments in freshwater ecosystems and are an important food source. This group will usually be more prevalent during the spring and will taper off as the water warms. Some genera associated with blooms that could cause taste or odor problems are Synedra, Tabellaria, Asterionella and Gomphonema. Chrysophytes are considered the golden-brown algae. Most species of chrysophyta are single-celled flagellates. Some species are colorless, but the majority are photosynthetic. Toxic blooms can be produced by several species. This group also is more prevalent in the spring when the water is cooler, and may move to the hypolimnetic water during the summer months. Some species may produce taste and odor problems but do not usually cause health problems. Some of the genera that may cause problems are Dinobryon, Synura, Mallomonas, and Uroglenopsis. The last group, dinoflagellates or pyrrhophytes, is one of the most important constituents of the marine and freshwater phytoplankton. The dinoflagellates are microscopic, usually unicellular, flagellated, often photosynthetic protists commonly 14

16 regarded as algae. Besides being important primary producers dinoflagellates are known for producing high levels of toxins, especially when they occur in large numbers. When these large numbers occur they are called red tides. The red tides can introduce non-fatal or fatal amounts of toxins into animals, especially shellfish that can then be eaten by humans who are affected by the toxins. Many of these toxins are quite potent and if not fatal, can still cause neurological damage. These deadly blooms usually do not occur in freshwater, however, two genera Peridinium and Ceratium have been know to cause fish kills in fresh water. The red tides appear to be more common in recent years and it is suspected that human input of phosphates to water bodies and warmer global temperatures may be adding to the increased frequency. As the growing season proceeds, a succession of algal communities typically occurs in a lake. A typical seasonal succession of lake phytoplankton would be: Diatoms dominate in the spring and autumn, green algae in mid-summer, and blue-green algae in late summer. Within a season there may be a shift from dominance by bloom forming bluegreen algae to one by diatoms or green algae. This shift may be due to changes in 1. CO 2 and ph, 2. Distribution of buoyant cells, and 3. Grazing by zooplankton (Cooke et al, 1993). Phytoplankton biomass usually tends to be high in the spring and early summer due to increasing water temperature and light availability, relatively high nutrient availability, and low losses to zooplankton grazing. As grazing pressure increases and nutrient availability declines from early to midsummer, algal biomass will usually decline. In the late summer and fall when the water column begins mixing and nutrient levels increase, the algal biomass may again increase. When looking at data for algal biomass versus total algal numbers sometimes the numbers have a high correlation, while other times they may not. This discrepancy may be due to algal communities that are composed of extremely high numbers of small algal cells or low numbers of extremely large algal cells. The size difference between species of algal cells can be quite large and these size differences may explain some of the discrepancies seen in the seasonal algal data for the monitoring program of the lakes. Another discrepancy that occurs is when comparing chlorophyll a to algal biomass. Chlorophyll a is present in all algae and therefore seems that there should be a direct correlation between the amount of Chlorophyll a sampled and the biomass of the algae. There are circumstances that cause this relation not to be a direct one. Not all alga cells may posses the same percentage of Chlorophyll a to dry algal biomass. Also, different species will produce different amounts of Chlorophyll a under the same conditions and certain environmental conditions may cause algal cells to produce more chlorophyll but not increase their numbers (of actual cells). Algal cells have also adapted themselves to be able to rapidly increase the amount of chlorophyll due to light conditions, so even in the same day (within hours) two samplings could produce different results if a storm event were to occur and decrease light levels directly or indirectly through more turbid water. This adaptation allows cells to have more available chlorophyll to utilize during the decreased light. All these variations in levels of chlorophyll production could cause an apparent discrepancy between chlorophyll levels and biomass of the phytoplankton communities. 15

17 Zooplankton Zooplankton are microscopic, crustacean organisms which free float and can filter up to the entire epilimnion in lake water grazing on detritus particles, bacteria and algae. In some lakes or reservoirs, the amount of algae in the open water may be controlled as much by zooplankton grazing as by the quantity of nutrients. The zooplankton feed on the algae or phytoplankton as well as on other smaller zooplankters. The most efficient grazers, which remove more particles, are the largest sized zooplankton species. These large zooplankton, however, are selectively eaten by fish, including the fry of most every fish species and the adults of planktivorous species such as shad, bluegill, pumpkinseed, perch and alewives. The zooplankton that feed on the algae are part of several orders including the Cladocera, Copepoda and Ostracoda. These are the small creatures commonly called waterfleas. There are several types of protozoans, rotifers and crustaceans that belong to those orders. Zooplankton populations will vary in a lake depending on temperature, food supply and level of predation by fish. The level of oxygen in the hypolimnion will directly affect the rate at which the zooplankton are consumed. If the DO is low in the hypolimnion, the zooplankton can not migrate to the lower waters and are forced to stay in the upper (epilimnion) waters where they are more readily accessible to fish predation. Just as the phytoplankton populations may vary within any given time period, so may the zooplankton populations. 16

18 2013 Climatological Conditions Climatological data from Dulles International Airport was used as an indicator of weather in the Reston area, as it has been since the environmental monitoring program for the Reston lakes began. Monthly average climatological conditions affect such factors as: extent of stratification, water temperature and observed oxygen depletion. Weather conditions in the days just prior to each sampling date can affect most of the other parameters measured. In other words, the monthly climatological conditions affect the long-term or seasonal trends of the lakes, while the weather conditions prior to a sampling date can affect the short term readings such as ph, conductivity and total phosphorous. A comparison of monthly rainfall and mean temperatures for 2013 with the 30-year averages is shown in Figure 1. Copies of the monthly climatological data summaries for Dulles International Airport from April through September 2013 are included in Appendix G. As mentioned in the synopsis, overall this season was slightly warmer than usual with below average rainfall. For the six month sampling period, the temperatures were a total of 1.6 degrees above normal and precipitation was 2.15 inches below normal. April was one of the months that was warmer and drier. A total of 2.3 inches of rain fell in April which was 1.2 inches below normal. The average temperature in April was 1.7 F above normal and the second week of April saw highs of almost 90 F. May had a little over 3 inches of rain which was still more than an inch below normal. The average temperature for the month was only 0.3 F above normal. In recent past, the weather seems to have dramatic swings in regard to rainfall and this season was no exception. April and May were over an inch below normal. By June precipitation was about 0.7 inches above normal and then in July 7.3 inches of rain fell which was 3.6 inches above normal. June was only about 1.0 F above average as only four days topped out at 90 F or higher. July is typically the warmest month of the season and this year followed suit. The monthly average was 1.6 F above normal for July, making it cooler than the last few years. August was cooler and drier. Rainfall totaled 2.0 inches, which was 1.6 inches below normal and temperatures were a little less than 2.0 F below normal. September remained dry as well with only a total of 1.6 inches of precipitation. Without the excess rain in July, the season was nearly six inches below normal. When combined with excessive heat, the dry conditions can lead to poor water quality on some water bodies. Luckily temperatures were moderate this season and overall the weather did not have a negative impact on the lakes at Reston. The antecedent rainfall for the five days prior to each sampling date and also the temperature and rainfall for the two weeks prior to each sampling date are summarized in Table 2. The 14 day antecedent conditions for the season showed some similarities to monthly totals with July experiencing ample rainfall and above average temperatures. 17

19 Every sampling date had some rain in the five day period prior to sampling. July was the highest with 1.15 inches. As mentioned previously, rainfall totals were quite varied during the 2013 season (Figure 1). Historically, antecedent temperatures (Table 2) show greater deviation throughout the season than rainfall. This year they were equally divergent. The average daily temperature departure from normal for two weeks prior to sampling ranged from 0.8ºF below normal in August to 5.9ºF above normal in April and the 14 day antecedent rainfall ranged from 5.67 inches in July to only 0.25 inches in July. 18

20 Temperature (Degrees F) Rainfall (Inches) Rainfall Data yr Avg 2 0 Apr May June July Aug Sept Month Temperature Data yr Avg Apr May June July Aug Sept Month Figure Climatological Data 19

21 Table 2 - Antecedent Weather Conditions for 2013 Reston Lakes Monitoring Program Sampling Date Rainfall (inches) for Days Preceding Sampling Total Rainfall (inches) Summary for Sampling Date and 14 Previous Days Temperature (average daily departure from normal, ºF) 16 Apr T* May T T T Jun T 0.00 T Jul Aug T T Sep * Trace precipitation amount Average for 15-day Period

22 Lake Anne Lake Anne was created in about 1962 by the impoundment of a Colvin Run tributary and is the oldest lake in Reston. Surface waters enter Lake Anne through two unnamed tributaries of Colvin Run and through immediate drainage from the land surrounding the lake. The northern tributary includes the overflow from Lake Newport, which is located about 1,200 feet upstream from Lake Anne. Lake Anne discharges into Anne s Run, which empties into Lake Fairfax, 0.6 miles downstream. Lake Anne has a surface area of 27.7 acres (11.2 hectares) and a watershed of 613 acres (247 hectares), including the lake surface area. The lake has a mean depth of 4.2 meters (13.7 feet) and a maximum depth of about 7 meters (23 feet). A summary of the ambient monitoring data is presented in Table 3. A summary of the 2013 results from the mixed layer for the trophic state variables along with phytoplankton and zooplankton results is presented in Table 4. Complete sampling results for 2013 are presented in Appendix A. For future reference, the figures showing historic data for conductivity, phosphorus, Secchi, chlorophyll, phytoplankton and zooplankton will show the current year and the 20 most recent years. Some of the figures were becoming very crowded and this will still give a visual historic perspective (this will apply to all the lakes). Chemical and Physical Parameters In general, temperature divergence was representative of a distinct thermocline throughout the entire season. The thermocline generally sets up between two to three meters during the sampling season. Most of this season, the thermocline occurred at 2.0 meters. August and September were the deepest at 3.0 meters. July showed the greatest difference between epilimnetic and hypolimnetic water temperatures. The epilimnetic average was 30.4ºC compared to 15.2ºC for the hypolimnetic average (Figure 2). In general, surface temperatures were equivalent to those since aeration ended, but overall lake temperatures still remain cooler than the years between 1997 and 2004 when aeration took place. The seasonal mean lake temperature was 17.2ºC, similar to the lower years in recent past. Dissolved oxygen percent saturation in the hypolimnion will almost always be below average since the aeration system has been shut down. The aeration system kept the lake mixed so that the mixed layer was deeper than it currently is. Thus the hypolimnion was small and represented fewer averaged values. Without aeration, Lake Anne s oxygen profile more closely resembles those of the other Reston lakes. Severe oxygen depletion in the hypolimnion traditionally begins around June and continues throughout the season. The seasonal percent saturation average for the hypolimnion and whole lake were both lower than their respective averages. They are however similar to the values observed before 1997 when the aeration system was not yet installed. Percent saturation in the epilimnion has undergone the reverse 21

23 transformation. The recent seasons have been higher than the years with aeration and the seasonal average of 96.7% this year was higher than the historic average. The ph levels in Lake Anne were equal to or slightly higher than long-term averages. The epilimnetic, hypolimnetic and whole lake seasonal means were 7.5, 6.6 and 7.0 and the respective averages are 7.0, 6.6 and 6.8. All of the conductivity levels were above average this year. Epilimnetic and hypolimnetic seasonal averages were 247 and 311µmhos/cm respectively (Figure 3a). The lake average was 283µmhos/cm compared to the historic value of 219µmhos/cm. Conductivities will continue to show greater deviation between the layers, once again due to the greater depth of the hypolimnion. During the years with aeration, the epilimnetic and hypolimnetic conductivities were much closer numerically. The conductivity levels were above average throughout the entire season, no one month was exceedingly high. Compared to past years, conductivities were somewhat elevated (Figure 3a). Historically, more rain has resulted in lower conductivities. The mean mixed layer total phosphorous concentration of 0.021mg/L observed this season was lower than the long-term average of 0.031mg/L. July had the lowest readings of the season (Figure 4). Higher phosphorus values in a season have not typically followed a distinct trend. Often, when a parameter is the highest for the season it can be linked to another parameter or natural event as a cause. However, the month with the greatest rainfall, highest chlorophyll or greatest algal density do not always correlate directly to the highest phosphorus reading. A few past seasons had seen a similar event though. The highest phosphorus reading occurred during a month with lower algal density and moderate chlorophyll values, but the greatest rainfall. This correlation did not hold true this year. Past records linked with the additional phosphorus sampling from 2002 (indicating over 90% of the phosphorus in Anne is organic) would tend to show that phosphorus in Lake Anne is due to internal loading. Comparing Table 2 (antecedent rainfall) and Table 4 (Lake Anne Monitoring Summary) illustrate the lack of correlation for phosphorus readings. June had the highest phosphorus reading for the season, less than a quarter of an inch of rain in the 5 days prior to sampling and low algal density. July had the lowest phosphorus reading of the season, over an inch of rain five days before sampling and the highest algal density of the summer. In past seasons, the rain may have brought phosphorus up from the hypolimnion or brought it in from external sources. Neither seems to have been the case this year though. For the past few years, Secchi depth has been close to the long-term average of 1.6 meters (Figure 5a). This year fell a little short of that mark at 1.2 meters. The Secchi depth will usually peak around 2 meters during the season and can fall below 1 meter at times. This season never reached 2 meters. June had the best clarity at only 1.5 meters (Figure 5). Other months had slight algal blooms, but clarity remained above 1.0 meter. 22